The photographic film used to make motion pictures comprises silver-halide crystals dispersed in an emulsion that is deposited in thin layers on a film base. The exposure and development of these crystals form the photographic image, which is made at the largest scale of discrete particles of silver. With color negatives, tiny blobs of dye exist on the sites where the silver crystals form after chemical removal of the silver following development. These small specs of dye form the ‘grain’ in color film. Grain occurs randomly in the resulting image because of the random formation of silver crystals on the original emulsion. Within a uniformly exposed area, some crystals develop by exposure, others not. Grain varies in size and shape. The faster the film (i.e., the greater the sensitivity to light), the larger the clumps of silver formed and blobs of dye generated, and the more they tend to group together in random patterns. The grain pattern is typically known as ‘granularity’.
The naked eye cannot distinguish individual grains, which vary from 0.0002 mm to 0.002 mm. Instead, the eye resolves groups of grains, which the viewer identifies as film grain. The larger the image resolution, the more likely the viewer will perceive the film grain. While clearly noticeable in cinema and high-definition images, film grain progressively loses prominence in Standard Definition Television (SDTV) images and becomes imperceptible in even smaller formats.
Typically, within the domain of video coding, efforts currently exist to improve the performance of the encoder to encode film grain at high bit-rates. Note that, since film grain becomes noticeable only in large image formats such as Standard Definition Television (SDTV), High Definition Television (HDTV) and above, the problem of film grain affects mainly professional encoders.
Present-day studies about film grain have targeted mainly photographic imaging applications (image edition, medical imaging, satellite imagery, astronomy and astrophotography, etc.). There currently exist software applications in the market place (Adobe After Effects, Photoshop, etc.) that provide solutions to this problem. In the domain of video coding, studies have addressed the desirability of removing film grain with the goal of improving the coding performance at mid and low bit-rates. However, the literature contains no specific strategies to encode film grain differently from other high frequency information, such as texture or contours.
In some sense, film grain constitutes the correlated noise inherent in the physical process of developing of photographic film. The presence of film grain denotes ‘real-world’ images contrast to ‘computer-generated’ material with no film grain. Images without film grain look like hyper-reality, simply too flat and too sharp to be real. For this reason, film grain remains a desired ‘artifact’ in most images and video material. However, because film grain arises from a random process affecting high frequencies, the encoding process typically affects film grain. Lossy encoders commonly achieve part of their compression gain by avoiding the transmission of the high frequencies in the DCT domain. Such frequencies are typically associated with noise, sharp edges and texture, but also with the film grain. Even at high bit-rates, film grain becomes difficult to preserve, and always at a high compression cost.
Thus, a need exists to alleviate the cost of encoding film grain without degrading the perceived quality of the displayed image(s).